In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, gloss, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendering rollers at the end of the process. A papermaking process typically has two types of directional control issues: machine direction (MD) control and cross direction (CD) control. MD refers to the direction of sheet travel and CD refers to the direction that is perpendicular to sheet travel.
A paper machine CD process is a large-scale two-dimensional system. The performance of a CD control, either a traditional single-input-single-output controller or an advanced model predictive controller, is highly dependent on the accuracy of CD alignment. In theory, CD alignment can be specified by using edge locations of paper web at both the actuator array side and the CD measurement array side and a CD nonlinear shrinkage profile. Both web edges and sheet shrinkage can change over time due to multiple causes, which result in misalignment issues. The causes include regular grade changes, variations in sheet tension between rolls, restraint during drying, and relative humility of the paper web itself. Current online methods that measure paper edges provide edge detectors to compensate for the sheet wander in closed loop however this technique is not able to detect the shape change of shrinkage profiles. Another online method measures CD shrinkage profile during the paper machine's normal operation. This technique uses wire marks, water marks, or felt marks, but these marks degrade the surface quality of the finished products.
When a CD process model alignment begins to differ from actual alignment, the CD control system is said to be misaligned. Misalignment of one third (⅓) of the actuator zone width can, in certain applications and circumstances, result in production loss as product fails to meet specifications. In addition, periodic variation patterns often referred to “picket fence” patterns in the actuator array are present. Actuator picketing causes product loss and degradation, wastes actuator energy and may cause physical damage to process equipment. When severe misalignment occurs, the CD controller must be detuned or switched off and realigned. Realignment typically entails an open-loop step test and automatic process identification and CD controller tuning. This realignment process disrupts normal paper production and is time consuming and tedious. Frequent and/or prolonged open-loop tests are undesirably as these lead to production inefficiency.
Systems that automatically map and align actuator zones to measurements points in sheetmaking systems have been developed. Some of these systems perform so-called “bump tests” by disturbing selected actuators and detecting their responses, typically with the CD control system in open-loop. The term “bump test” refers to a procedure whereby an operating parameter on the sheetmaking system, such as actuator setpoints of a papermaking machine, is altered and changes of certain dependent variables resulting therefrom are measured. Prior to initiating any bump test, the papermaking machine is first operated at predetermined baseline conditions. By “baseline conditions” is meant those operating conditions whereby the machine produces paper of acceptable quality. Typically, the baseline conditions will correspond to the current process conditions in open loop. Given the expense involved in operating the machine, extreme conditions that may produce defective, non-useable paper are to be avoided. In a similar vein, when an operating parameter in the system is modified for the bump test, the change should not be so drastic as to damage the machine or produce defective paper. After the machine has reached steady state or stable operations, certain operating parameters are measured and recorded. Sufficient number of measurements over a length of time is taken to provide representative data of the responses to the bump test.
For example, U.S. Pat. No. 5,400,258 to He discloses a standard alignment bump test for a papermaking system wherein an actuator is moved and a scanning sensor reads its response and the alignment is identified by the software. U.S. Pat. No. 6,086,237 to Gorinevsky and Heaven discloses a similar technique but with more sophisticated data processing. Specifically, in their bump test the actuators are moved and technique identifies the response as seen by the scanner.
More recent approaches to monitoring and identifying CD alignment include U.S. Pat. No. 6,564,117 to Chen et al which describes a process whereby the CD profile of a web of material be produced is monitored and controlled. This passive closed-loop identification technique cannot identify severe misalignments and cannot run in open-loop. U.S. Pat. No. 7,128,808 to Metsala et al. describes a method for identifying mapping that employs a mapping model that takes the linear and non-linear shrinkage of paper web into account. This open-loop nonlinear shrinkage identification algorithm requires that the shrinkage profile be divided into three sections. U.S. Pat. No. 7,459,060 to Stewart describes closed-loop identification of CD controller alignment but this approach cannot handle actuator constraints and cannot be applied to multivariable CD control systems. Finally, U.S. Pat. No. 7,648,614 to Tran et al. describes an elaborate method of controlling CD mapping in a web that requires generating at least two analysis rule profiles from data. The technique requires much testing and computer memory.